Diamonds are nothing more than chunks of coal that stuck to their jobs

Malcolm Forbes

Very high temperatures and pressures are required for the formation of natural diamonds.
Beneath the Earth's mantle about 90 miles (150 kilometers) where temperatures are at least 2000 degrees Fahrenheit (1050 degrees Celsius)
are where these conditions occur in limited zones.
This critical temperature-pressure environment for diamond formation and stability is thought to be present primarily in the mantle
beneath the stable interiors of continental plates.
Therefore it is not present globally.